Europium(III) Meets Etidronic Acid (HEDP): A Coordination Study Combining Spectroscopic, Spectrometric, and Quantum Chemical Methods

Abstract

Etidronic acid (1-Hydroxyethylidene-1,1-diphosphonic acid, HEDP, H₄L) is a proposed decorporation agent for U(VI). This paper studied its complex formation with Eu(III), an inactive analog of trivalent actinides, over a wide pH range, at varying metal-to-ligand ratios (M:L) and total concentrations. Combining spectroscopic, spectrometric, and quantum chemical methods, five distinct Eu(III)-HEDP complexes were found, four of which were characterized. The readily soluble EuH₂L⁺ and Eu(H₂L)₂^- species with log β values of 23.7 ± 0.1 and 45.1 ± 0.9 are formed at acidic pH. At near-neutral pH, EuHL⁰_s forms with a log β of ~23.6 and, additionally, a most probably polynuclear complex. The readily dissolved EuL^- species with a log β of ~11.2 is formed at alkaline pH. A six-membered chelate ring is the key motif in all solution structures. The equilibrium between the Eu(III)-HEDP species is influenced by several parameters, i.e., pH, M:L, total Eu(III) and HEDP concentrations, and time. Overall, the present work sheds light on the very complex speciation in the HEDP-Eu(III) system and indicates that, for risk assessment of potential decorporation scenarios, side reactions of HEDP with trivalent actinides and lanthanides should also be taken into account.

Keywords: ATR-FT-IR; DFT; ESI-MS; NMR; TRLFS; complexation; lanthanides; speciation.

Figure 1

Generic structures of HEDP’s protonation species together with the abbreviations used throughout this work.

Figure 2

The soluble fraction in the Eu(III)–HEDP system as determined by ICP-MS in supernatants of TRLFS pH-titration series (A,C) and HEDP concentration series at constant pH 2, 5, and 11.5 (B) at 10⁻⁶ M Eu(III), without background electrolyte and at room temperature (colors are based on the pH from acidic = red to alkaline = blue).

Figure 3

Steady-state luminescence spectra (A,D), luminescence decay curves (B,E) and speciation (C,F) of 10⁻⁵ M Eu(III) at pH 2 (A–C) and pH 5 (D–F), I = 0.1 M (NaCl) and (25 ± 1) °C in dependence on the HEDP concentration (steady-state spectra are normalized to the ⁷F₁ band area, and decay curves are normalized to the luminescence intensity at t = 0).

Figure 4

Steady-state luminescence spectra (A), luminescence decay curves (B) and speciation (C) of 10⁻⁶ M Eu(III) at equimolar M:L, without background electrolyte and at (25 ± 1) °C in dependence on the pH (steady-state spectra are normalized to the ⁷F₁ band area, and decay curves are normalized to the luminescence intensity at t = 0).

Figure 5

DFT calculated solution structures of the EuH₂L⁺ (A), Eu(H₂L)₂⁻ (B), EuHL⁰_s (C), and EuL⁻ (D) complexes.